Diamond coated antireflective window system and method

ABSTRACT

A system and method for diamond based multilayer antireflective coating for optical windows are provided. An antireflective coatings for optical windows may include an optical grade silicon substrate, a first polycrystalline diamond film on the silicon substrate, a germanium film on the first polycrystalline diamond film, a fused silica film on the germanium film; and a second polycrystalline diamond film on the fused silica film. A method of fabricating a diamond based multilayer antiretlective coating may include the steps of cleaning and seeding an optical substrate, forming a first diamond layer on the optical substrate, forming a germanium layer on the first diamond layer, forming a fused silica layer on the germanium layer, cleaning and seeding the germanium layer, and forming a second diamond layer on the germanium layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/301,746, filed Mar. 1, 2016, which is fully incorporated herein byreference.

BACKGROUND Field

This invention is generally related to systems and methods forantireflective coatings for optical windows, and more particularly to asystem and method for providing diamond based multilayer antireflectivecoating for optical windows.

Background

Diamond possesses favorable theoretical semiconductor performancecharacteristics, including the possibility of creating transparentelectronics, including those related to optical windows. However,practical diamond based semiconductor device applications for opticalwindows remain limited.

SUMMARY

Disclosed herein is a new and improved system and method for diamondbased multilayer antireflective coating for optical windows. Inaccordance with one aspect of the approach, a system for antireflectivecoatings for optical windows may include an optical grade siliconsubstrate, a first polycrystalline diamond film on the siliconsubstrate, a germanium film on the first polycrystalline diamond film, afused silica film on the germanium film; and a second polycrystallinediamond film on the fused silica film.

In another approach, a method of fabricating a transparent electronicdevice, may include the steps of cleaning and seeding an opticalsubstrate, forming a first diamond layer on the optical substrate,forming a germanium layer on the first diamond layer, forming a fusedsilica layer on the germanium layer, cleaning and seeding the germaniumlayer, and forming a second diamond layer on the germanium layer.

Other systems, methods, aspects, features, embodiments and advantages ofthe system and method disclosed herein will be, or will become, apparentto one having ordinary skill in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional systems, methods, aspects, features, embodiments andadvantages be included within this description, and be within the scopeof the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

It is to be understood that the drawings are solely for purpose ofillustration.

Furthermore, the components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of thesystem disclosed herein. In the Figures, like reference numeralsdesignate corresponding parts throughout the different views.

FIG. 1 is schematic diagram of an exemplary monolithically integratedantireflective coating thin film structure.

FIG. 2 a block diagram of an embodiment of a method for fabricating amonolithically integrated antireflective coating thin film structure,such as the structure of FIG. 1.

DETAILED DESCRIPTION

The following detailed description, which references to and incorporatesthe drawings, describes and illustrates one or more specificembodiments. These embodiments, offered not to limit but only toexemplify and teach, are shown and described in sufficient detail toenable those skilled in the art to practice what is claimed. Thus, forthe sake of brevity, the description may omit certain information knownto those of skill in the art.

Prior medium wavelength infrared antireflective coatings may suffer fromdelamination, degradation, and fluctuating optical transmissivity.Nanocrystalline diamond may provide high transmissivity infrared windowswith high reliability. Semiconductor grade nanocrystalline diamondmaterial is effective in optical display applications, particularly intandem with fused silica thin films. Multilayer film synthesis providesnanocrystalline diamond based antireflective coatings fabrication whichis beneficial for use in medium wavelength infrared applications, forexample in wavelength applications from 3.0 micrometers to 5.0micrometers.

Single layer antireflective coatings may minimize the reflectance, butmay reduce it to zero only for a certain value of film refractive index.A multi-layer antireflective coating deposited on a substrate may reducethe reflectance from the substrate to zero for a certain wavelengthrange. Optical grade silicon may have an index of refraction (n) ofapproximately 3.4. Lower value index of refraction diamond (n=2.4) mayhe incorporated with optical materials of higher and lower indices ofrefraction, such as germanium (n=4.0) and fused silica (n=1.4),respectfully, to produce near zero reflectance at infrared wavelengthsfor certain values of film thicknesses. Utilizing optical designprograms, such as open-source software for the design, optimization, andsynthesis of optical filters, for example, Openfilters, with inputsoptimized for the medium wavelength infrared range, high transmittanceat a variety of critical angels may be realized via a 4-layerantireflective coating consisting of an optical substrate Silicon, afirst antireflective coating film layer of nanocrystalline diamond, asecond antireflective coating film layer of germanium, a thirdantireflective coating film layer of fused silica, and a antireflectivecoating film layer of nanocrystalline diamond with film thicknesses ofapproximately 104 nm, 595 nm, 432 nm and 115 nm, respectfully. Further,utilizing this modality, the hydrophobicity and scratch resistance ofdiamond may enhance the performance of connecting fused silica layer, aswater accumulation is known to cause absorption of light in fused silicasystems.

Diamond layers may provide advantageous such as ultra-hardness,scratch-resistance, high thermal conductivity, hydrophobicity, chemicaland biological inertness, amongst others. In part, due to difficulty infabricating high quality thin films cost efficiently, diamond hastraditionally not been used for infrared optical window applications dueto diamond's sharp and semi-sharp absorption spectra in the mediumwavelength infrared region. However, through novel multilayercombination with optical materials of larger and smaller refractiveindex values, it is now possible to capture the favorable materialproperties of diamond in antireflective coatings for medium wavelengthinfrared optical window applications. The disclosed systems may providea more rugged system for various applications, such as for battlefieldapplications. This disclosure provides several preferred embodiments offabrication, however, the performance characteristics and materialscharacteristics described in this application are not necessarilyperformance bounds or limitations of the invention. These disclosuresmerely demonstrate some aspects of the invention that have presentlybeen tested.

FIG. 1 shows a schematic diagram of an exemplary monolithicallyintegrated antireflective coating thin film structure 100. Structure 100may include a substrate 102, a first diamond thin film 104, a germaniumfilm 106, a fused silica film 108, and a second diamond thin film 110.Substrate 102 may be an optical grade substrate, such as, but notlimited to an optical grade silicon substrate. Diamond thin films 104and 110 may be polycrystalline diamond thin films. Film structure 100 isshown as an exemplary composite film structure and exemplary sequencefor a diamond based multilayer antireflective coating for infraredoptical windows.

In one embodiment, the second diamond thin film 110 may be 115nanometers, the fused silica film 108 may be 432 nanometers, thegermanium film 106 may be 595 nanometers, and the first diamond thinfilm 104 may be 104-105 nanometers. In another embodiment, the firstpolycrystalline diamond film 104 may be between 100 and 110 nanometers,the germanium film 106 may be between 590 and 600 nanometers, the fusedsilica film 108 may be between 427 and 437 nanometers, and the secondpolycrystalline diamond film 110 is between 110 and 120 nanometers

FIG. 2 shows a block diagram of an embodiment of a method 200 forfabricating a fabricating a diamond based multilayer antireflectivecoating, such as exemplary monolithically integrated antireflectivecoating thin film structure 100. Fabrication of such a system can berealized utilizing a combination of chemical vapor deposition, physicalvapor deposition, and reactive ion etching systems.

Method 200 may include a first step 202 of selecting an opticalsubstrate, such as an optical grade silicon wafer, for example,substrate 102. Method 200 may include a second step 204 of cleaning andseeding the optical grade silicon wafer of first step 202. For example,step 204 may include exposing the optical grade silicon wafer to an acidcleaning mixture, such as (4:1 H₂SO₄/H₂O₂, H₂O₂, 5:1:1 H₂O/H₂O₂/HCl).Step 204 may also include seeding the optical grade silicon wafer with ananoseed solution mixture and ultrasoniced in alcohol solution topromote nucleation and film agglomeration.

Method 200 may include a step 206 of forming a first diamond layer. Step206 may include chemical vapor deposition growth and etching of thediamond layer. Step 206 may include exposing the wafer of step 204 to amethane, argonne, and hydrogen plasma gas mixture in a chemical vapordeposition system to produce the a thin nanocrystalline diamond film,for example, the first diamond thin film 104. In the event the diamondgrowth is beyond the target thickness, reactive ion etching via anargonne and oxygen mixture may produce bulk planarized uniform diamondfilms.

Method 200 may include a step 208 of forming a germanium layer. Step 208may include sputtering physical vapor deposition and etching. Step 208may include high purity germanium targets properly loaded into amagnetron sputtering physical vapor deposition system to producegermanium film, such as, for example, germanium film 106. Step 208 mayinclude ion milling in the event the germanium growth is beyond thedesired thickness.

Method 200 may include a step 210 of forming a fused silica film. Step210 may include sputtering physical vapor deposition and etching. Step210 may include a high purity fused silica target loaded into asputtering physical vapor deposition system to produce fused silicafilms, such as, for example, the fused silica film 108. Step 210 mayalso include ion milling in the event of fused silica growth is beyondthe desired thickness.

Method 200 may include a step 212 of cleaning and seeding. Step 212 mayinclude cleaning the product of prior steps with an ultrasonic bath.Step 212 may include seeding with nanocrystalline diamond seed solutionand ultrasoniced with an alcohol solution mixture

Method 200 may include a step 214 of forming a second diamond layer.Step 214 may include chemical vapor deposition growth and etching of thediamond layer to produce a nanocrystalline diamond film, for example,the second diamond thin film 110.

The commercially available optics design software suite, Open Filters,was utilized to simulate the transmittance of the proposed system.Embodiments of the system and method were shown to have beneficialtransmissivity in the infrared spectrum ranges between 3700 nanometersand 4900 nanometers in wavelength with transmittance shown to be inexcess of 9.4% at critical incident angles of 0°, 15°, 30°, and 45°.

Structure 100 and method 200 may incorporate systems and methodspreviously disclosed and described in U.S. Patent Publication No.2013/0026492, by Adam Khan, published on Jan. 31, 2013, U.S. Pat. No.8,354,290, issued to Anirudha Surnant, et al, on Jan. 15, 2013; U.S.Pat. No. 8,933,462, issued to Adam Khan on Jan. 13, 2015; U.S. Patent.Publication No. 2015/0206749, by Adam Khan, published on Jul. 23, 2015,and U.S. Patent Publication No. 2015/0295134, by Adam Khan, et al,published on Oct. 15, 2015, all of which are fully incorporated hereinby reference.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or variant described hereinas “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or variants. All of the embodimentsand variants described in this description are exemplary embodiments andvariants provided to enable persons skilled in the art to make and usethe invention, and not necessarily to limit the scope of legalprotection afforded the appended claims.

The above description of the disclosed embodiments is provided to enableany person skilled in the art to make or use that which is defined bythe appended claims. The following claims are not intended to be limitedto the disclosed embodiments. Other embodiments and modifications willreadily occur to those of ordinary skill in the art in view of theseteachings. Therefore, the following claims are intended to cover allsuch embodiments and modifications when viewed in conjunction with theabove specification and accompanying drawings.

1. A method of fabricating a diamond based multilayer antireflectivecoating, the method including the steps of: cleaning and seeding anoptical substrate; forming a first diamond layer on the opticalsubstrate; forming a germanium layer on the first diamond layer; forminga fused silica layer on the germanium layer; cleaning and seeding thegermanium layer; and forming a second diamond layer on the germaniumlayer.
 2. The method of claim 1, wherein the optical substrate is anoptical grade silicon substrate.
 3. The method of claim 1, whereinforming the first diamond layer includes a chemical vapor deposition. 4.The method of claim 1, wherein forming the germanium layer includes asputtering physical vapor deposition.